best voltage to send to classic for most power/efficiency

[kauaisolarman]

hello again

just as the title says what is the best/most efficient voltage to send to the charge controller from the PV array?

as far as i can interpert from the manual and the online brochures the best voltage for 12/24/48 volts is 70 volts.

can someone verify this for me.

i know they can take up to the voltage that they are designed for i.e. classic 150 = 150 volts max, but, what is the best voltage to send to the controller so it produces the most power out of that string/array?

thanks again

[Vic]

hello again

just as the title says what is the best/most efficient voltage to send to the charge controller from the PV array?

as far as i can interpert from the manual and the online brochures the best voltage for 12/24/48 volts is 70 volts.

can someone verify this for me.

i know they can take up to the voltage that they are designed for i.e. classic 150 = 150 volts max, but, what is the best voltage to send to the controller so it produces the most power out of that string/array?
thanks again

Hello,  Kauai..,

Here is a Link to the Classic String Sizer:
http://www.midnitesolar.com/sizingTool/displaySizing.php

In general,  for Classics,  the STC String Vmp should be about 130% of the maximum voltage that the battery bank will ever need.  This highest voltage for Flooded batteries would be in EQ (as stated in the foot notes from the results of the Sizer).

Generally,  the lowest STC String Vmp that satisfies the above,  should be the most efficient,  and generate the least heating in the Charge Controller (CC).

BTW, a 70 volt STC string Vmp will be too low to fully-charge,  or EQ most any 48 V battery bank  --  perhaps a 48 V  Gel battery at room temperature might be happy   ...

Often,  because many PVs come in certain Vmps ranges(like those of 60-cell,  and 72 cell PV modules),  one cannot chose what appears to be the perfect STC input voltage to an MPPT CC at a reasonable price.   Most systems do not really need to be perfect,  but,  perfection is a nice starting goal.

FWIW,   Vic

[kauaisolarman]

thank you for the input vic.

why does the classic 200-SL have it producing the most power for a 70V PV array for charging a 48V battery bank if 70V is not suficcient to charge a 48V battery bank?

i see this from the chart on page 8 from the manual
http://www.midnitesolar.com/pdfs/Classic-SL-REV-2056.pdf

it shows the classic 200 graph producing the most watts at 70V for the 48 volt battery bank (4550watts)  where 120v only produce (4080watts) to the same battery voltage.  from the graph it seems that 70V is ideal array voltage for charging all battery voltages from 12-48V..

am i not understanding the graph correctly?

i have a 48V bank i intend to charge and i have panels with Vmp of 36.5.  I have 6 panels and was going to run then in 2 series (73Vmp), but if that is not enough for charging my battery bank i will run them in 3 series (109.5 Vmp).

if im seeing correct running pv input at 109.5 Vmp would be a 5-10% loss versus running at 73Vpm.

what shold i do?

thanks again for all the help.  trying to learn things as i go.

[Vic]

Hi kauai..,

Do not know the nature of the batteries that you will be using on this system.  IF they are Flooded batteries,  IMO a Vmp of 70.0 V is just too low for efficient use of most any MPPT CC.

EDIT:   >Should add,  that from the Graphs of voltage verses Output current and power,  the Legend does say,  "MPPT Input Operating Voltage",  which would require a higher String STC voltage (which comes from Flash Tube tests at 25 degrees C Cell temperatures.  Most common PVs have  NOCTs (Normal Operating Cell Temperature)  of about 46 - 48 C,  and this value is at somewhat reduced sun intensity (Irradiance),  and cool ambinets,    and with some wind.   The String Sizer,  and my references were to the String STC voltage that comes from the STC Vmp of each PV in a string.<

But,  again IMO,  73 V string STC Vmp is considerably better with FLAs than would be 70.0 volts.

With FLAs,  particularly,  ones which are cool/cold  (which would raise the required charge/EQ voltage)  you could run into a situation where you could TRY to EQ the battery,  but may well not be able to reach a real EQ voltage.

Your temperatures are probably moderate,  so your batteries may never be that cold,  but on warm/hot days,  where the actual string voltage will be reduced from the PVs being hot,  this could mean that the Classic will not be able to operate well at that input voltage,  particularly during Absorb (EQing would be even worse).

So   a low Vin may make the CC run cooler,  and therefore might be theoretically more efficient,  there could be a considerable loss in power Production, due to the CC not having enough headroom to find a Vmp that produces the maximum power available from the PVs at that time.

Please DO use the MN String Sizer,  Linked above.   Please DO look at the Notes under the results from the Sizer.   You will find the strong suggestion from MN,  that the STC String Vmp should be at least 130%  of the highest charge/EQ voltage that you will ever need.

IF your batteries are Flooded,  this could place you in 83-ish volt range,  or perhaps a bit higher.   This really means,  that with a 48 V battery bank,  one might wish to use 60-cell PVs,  in strings of three.

You would probably find that strings of two of those 72 cell PVs will generally seem to work fairly well,  but on hot days,  there could be trouble finishing Absorb,  particularly if the batteries are cool.

ALL just my opinions,  FWIW,   Vic

[kauaisolarman]

thanks very much for the reply and knowledge vic.

after thinking about everything you are correct and i will go with 3 panes series together for 109.5 vmp for my 58V battery bank.

absorb voltage for my bank is 57.6V and float is 54.0 volts

no equalization charge needed for these type of batteries.